1. Field of the Invention
This invention relates to DC-to-DC converters, DC-to-AC inverters and AC-to-DC converters. The major characteristic of this power conversion technique is that transfers the power to the secondary continuously, and the switching elements switch at zero voltage.
2. Description of the Prior Art
There is a continuing industry demand for increasing power density, which means more power transferred in a given volume. A method for increasing the power transfer through the converter is to increase the switching frequency in order to minimize the size of magnetic and the capacitors. Using prior art topologies such as forward or flyback, which employ "hard" switching techniques, makes high frequency operation less efficient. The switching losses associated with switching elements, which turn on when there is a voltage across them, are proportional with the switching frequency. An increase in switching frequency, leads to an increase in switching losses and an increase in level of electromagnetic interference (EMI). In order to overcome limitations in switching speeds, the prior art has devised a new family of resonant and quasi-resonant converters. In the case of quasi-resonant converters, the prior art technique consists of shaping the current or voltage to become half sinusoidal and to perform the switching when the current or voltage reaches zero. The reactive elements which contribute to shaping the current or voltage are part of the basic circuit and are considered undesirable in classic topologies. An example of one such circuit can be found in Vinciarelli, "Forward Converter Switching at Zero Current", U.S. Pat. No. 4,415,959. The technique utilized by Vinciarelli consists of adding a resonant capacitor across the fly wheeling diode to create a resonant circuit in combination with the leakage inductance of the transformer. During the ON time of the main switch, a current charges the resonant capacitor. When the current reaches zero, the main switch turns OFF in the primary of the transformer. The output inductor discharges the resonant capacitor, transferring the energy to the load. This topology eliminates part of switching losses which allows the converter to run at a high frequency. However, this topology exhibits several drawbacks which limit its utilization to power under 200 W. The peak current in such a quasi-resonant converter is significantly large if a large input voltage range is required. The energy is transferred in stages from the input to the resonant capacitor and then from the resonant capacitor to the output. Due to the fact that the main switch in the primary turns ON at zero current and non zero voltage, the energy contained in the output capacitance of the switch is dissipated. The output power is varied by varying the frequency. A certain amount of energy is transferred from the input to the output at every cycle and when the power requirements are high, the repetition frequency is correspondingly high. However, the modulation in frequency does not allow significant decrease of the output filter size. A large electromagnetic interference (EMI) filter is required to avoid beat frequency problems between the units, if two non-synchronized units are used together.
Another family of quasi-resonant converters which switch at zero voltage is described by F. C. Lee in High Frequency Power Conversion International Proceedings (April 1987), Intertec Communications, Ventura, Calif. These prior art circuits operate similarly to those described above with the exception that the main switch turns ON and OFF at zero voltage. This has the advantage of eliminating the losses caused by the discharged of the capacitance of the switch at turn ON and also decreases the driving current utilized in the MOSFET switch due to the elimination of the Miller effect. However, the voltage across the main switch and the frequency modulation which is required for controlling the output power makes this topology unattractive.
An additional group of quasi-resonant converters includes the multi-resonant converters such as were described at the High Frequency Power Conversion International Proceedings (May, 1988), Intertec Communications, Ventura, Calif. While operating similarly to other quasi-resonant topologies, a secondary resonant circuit is employed to decrease the stress across the output rectifier and to reduce frequency swings over various input-output conditions of operation.
Another drawback associated with prior art power conversion technique is the fact that the power is transfer to the secondary during a portion of the cycle. Common prior art topology such as forward, transfers the power to the secondary during the ON time of the main switch. In the case of flyback converter the power is transferred to the load during OFF time of the main switch. These topologies require large output filters in order to smooth the power transfer to the load.
What is needed is a converter which operate at constant frequency, modulating the power by varying the duty cycle, the current and voltages on the switching elements are square-wave to decrease the current and voltages stress, the transitions are done at zero voltage conditions, and the power is transferred to the output, both during the ON time and OFF time.